Chain-breaking process

With this general sequence of reactions as our proposed mechanism, we are now prepared to use the corresponding rate expressions and our standard assumptions to show that rate expressions with reaction orders of 1/2, 1, and 3/2 can be derived. The form of the rate expression that results is an indication of the nature of the chain breaking process. 

In order for the overall rate expression to be 3/2 order in reactant for a first-order initiation process, the chain terminating step must involve a second-order reaction between two of the radicals responsible for the second-order propagation reactions. In terms of our generalized Rice-Herzfeld mechanistic equations, this means that reaction (4a) is the dominant chain breaking process. One may proceed as above to show that the mechanism leads to a 3/2 order rate expression. 

Based on the data collected in this section, one must conclude that the addition of radicals to dienes is certainly rapid enough to compete against the typical chain-breaking processes and that especially the addition of electrophilic radicals to polyenes appears to bear significant potential. Terminally substituted polyenes are likely to be unsuitable for radical addition reactions due to their lower addition rates and to undesirable side reactions. 

The total rate of chain breaking is the sum of the rate of termination and the rates of all other chain-breaking processes, Rr ... 

The quantum yield for the chain-breaking process in a polymer molecule is given by... 

Solvent polarity and temperature also influence ihe results. The dielectric constant and polarizability, however, are of little predictive value for the selection of solvents relative to polymerization rates and behavior. Evidently evety system has to he examined independently. In cationic polymerization of vinyl monomers, chain transfer is the most significant chain-breaking process. The activation energy of chain transfer is higher than that of propagation consequently, the molecular weight of the polymer increases with decreasing temperature. 

It is evident that all these mechanical methods are non-selective and that the block copolymer can undergo itself chain scission so that the copolymers would be formed from multiple blocks linked together. In the extreme case, when the chain breaking process is maintained during a sufficient long time a random copolymer would even be obtained. On the other hand the products which are formed are most likely mixtures of block and graft copolymers indeed chain transfer which must occur in... 

The reduction and oxidation of radicals are discussed in Chapter. 6.3-6.5. That in the case of radicals derived from charged polymers the special effect of repulsion can play a dramatic role was mentioned above, when the reduction of poly(U)-derived base radicals by thiols was discussed. Beyond the common oxidation and reduction of radicals by transition metal ions, an unexpected effect of very low concentrations of iron ions was observed in the case of poly(acrylic acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly reproducible, but when the glass ware had been washed with EDTA to eliminate traces of transition metal ions, notably iron, from its surface, results became reproducible. In fact, the addition of 1 x 10 6 mol dm3 Fe2+ reduces in a pulse radiolysis experiment the amplitude of conductivity increase (a measure of the yield of chain scission Chap. 13.3) more than tenfold and also causes a significant increase in the rate of the chain-breaking process. In further experiments, this dramatic effect of low iron concentrations was confirmed by measuring the chain scission yields by a different method. At present, the underlying reactions are not yet understood. These data are, however, of some potential relevance to DNA free-radical chemistry, since the presence of adventitious transition metal ions is difficult to avoid. 

The equilibrium between active species and dormant species is also helpful in preventing chain-breaking processes, such as termination and transfer reactions. 

Since it is a chain breaking process, chain transfer leads to lower molecular weights. However, its effect on the polymerization rate depends on the values of the rate coefficients. In the case where the rate limiting step is a radical transfer to the CTA (T), there is a decrease in M but no effect on monomer conversion rate. If the radical transfer from T to monomer is much slower than the transfer to T, then inhibition can occur, which slows down the reaction and leads to formation of longer chains. 

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